We have worked out the principal isolation procedures, and will now prepare the chicken red cell R1 antigen and the plasma membrane receptor for an avian B leukosis-sarcoma virus in substantial quantities. We intend to obtain these materials in highly active and homogeneous form and will begin to characterize them immunochemically and chemically. This will also be helpful in precise determination of the cross-relation which we have observed between these chicken cell surface structures, the MN and the major TA3 tumor glycoprotein.